Some colour about desktop (3)
I believe the answer is:
red
'some colour' is the definition.
(red is a kind of colour)
'about desktop' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'desktop' becomes 'd' (the top letter of 'desk' - in a down clue).
're'+'d'='RED'
(Other definitions for red that I've seen before include "Colour of 1 across" , "Colour of a cardinal's hat" , "Colour for danger" , "Ruby" , "A colourful communist" .)